Semiconductor manufacturers are continuously striving to produce faster and more complex integrated circuits. The most direct way of achieving this is to reduce the dimensions of a semiconductor circuit, thereby decreasing the gate (transistor) delay. However, as the dimensions of circuits are reduced, in particular as the width of conducting interconnects contained in the circuit decreases, the electrical resistance of the interconnect is proportionately increased. This causes an increase in the gate delay and the loss of electrical energy through heat production.
In order to offset the side-effects of reducing circuit dimensions, materials with low bulk resistivity are used to manufacture the interconnects of the circuits. Copper is one such material. Its use is favoured because, as well as having low bulk resistivity, it also has a relatively high heat conductivity and melting point. However, like some other metals that can potentially be used as the material in interconnects, copper suffers from poor adhesion to dielectrics and copper has a tendency to be degraded by electromigration. These drawbacks may be overcome by depositing a thin capping layer, typically comprising an alloy, on the copper interconnects. The capping layer is also known as a passivation layer.
Techniques used to deposit the capping layer onto the interconnect are generally selective in nature, so that they deposit the capping layer onto the interconnect but not onto the dielectric surface. One such technique is electroless deposition. However, the selectivity and effectiveness of this deposition of the capping layer depends on the surface of the substrate being very clean. In other words, impurities or contaminants on or in the surface of either the dielectric layer or the interconnect result in reduced selectivity and effectiveness of the deposition.
One possible contaminant is an oxide layer at the surface of the interconnect. This oxide layer is usually spontaneously created during the formation of the interconnects. For example, a layer of copper oxide is formed at the surface of copper interconnects during Chemical Mechanical Polishing (CMP) of the interconnect, partly spontaneously through reaction with the atmosphere and partly a result of the chemical treatment during CMP.
In the past, any oxide contaminant formed prior to deposition of the capping layer has usually been removed by wet treatment of the surface of the substrate. The wet treatment typically involves washing with an aqueous acidic solution having a low pH, sometimes containing additives such as surfactants. This solution can successfully remove the layer of oxide at the surface to expose the surface of the un-oxidized interconnect. The acid treatment also removes other contaminants from the surface, such as metal residues and organic residues.
However, the inventors of the present invention have found that the current acid etching process is aggressive and it is not wholly selective for removing just the oxide; instead, a small but significant amount of the non-oxidized conducting interconnect is also etched. This is demonstrated in FIG. 1, which shows an interconnect after treatment with a conventional aggressive acid cleaning solution. This Figure demonstrates that conventional cleaning can result in the formation of a recess at the surface of the interconnect where material from the surface (both oxide and non-oxide) has been removed from the surface.
This formation of recesses results in a decrease in the cross-sectional area of the interconnect, thereby increasing its resistance. This in turn results in an increased amount of heat produced by the increased electrical resistance, and consequently increases the gate delay and rate of electromigration. Ultimately, this results in the reduced performance and a decreased lifetime of the transistor.
In view of the drawbacks of a conventional acid wash, the inventors have looked for improved methods of treating an oxidized surface prior to deposition of a capping layer. One potential method is described in US 2007/037389. This describes the treatment of the surface of a copper interconnect with an ethanol vapour prior to deposition of a capping layer. It is thought by the present inventors that this method results in the reduction of copper oxide at the surface of the interconnect by the following chemical reactions:12CuO(s)+CH3CH2OH(g)→6Cu2O(s)+2CO2(g)+3H2O(g) then:6Cu2O(s)+CH3CH2OH(g)→12Cu(s)+2CO2(g)+3H2O(g) 
Accordingly, this method does not etch away the unwanted copper oxide; instead, it reduces the copper oxide into metallic copper. As a result, the present inventors have found that no recess tends to be formed by this method of etching.
However, the present inventors have also found that, because this method does not etch away any oxide but instead simply deposits the reduced metal onto the surface, it has a tendency only to treat the very top layers of metal oxide. This is because the reduced metal, once it has been deposited, in effect acts as a barrier preventing the reduction of the remaining part of the layer of metal oxide. This reduces the homogeneity of the interconnect and it may result in an increased level of electromigration in the interconnect. Any remaining copper oxide, even if not directly at the surface of the substrate, may also reduce the effectiveness and selectivity of the deposition of the capping layer.